It is well known to provide circuitry for measuring rms value of a bipolar wide band audio input signal, as a logarithmic function. An example of such a circuit is that of U.S. Pat. to Blackmer, No. 3,681,618. It is also well known to measure peak values of an audio signal which varies rapidly in amplitude over extremely wide ranges. Such systems usually provide linear responses, or if they provide logarithmic responses are extremely complex and therefore costly. It is an object of the present invention to provide an electronic peak rectifying system which is economical to fabricate, requiring only four discrete rectifying junction devices, some or all of which may be transistors, one operational amplifier and an inverter, yet which provides a logarithmic response representing peak voltage at the input of the system. Such a peak rectifying system finds utility in a meter as well as in other applications.
The operational requirements of a peak reading meter are quite distinct from those of an rms meter. A peak reading meter must respond rapidly to a rapidly increasing voltage, but must hold its reading if the voltage rapidly decreases, i.e., it is not enough to integrate voltages, or to average voltages.
The present system employs a logarithmic amplifier which is in a broad sense known, as in the Blackmer patent, but which includes improvements over the latter. The system also includes a simple device, requiring only two rectifying junction devices, which may be transistors and an inverter which can charge a capacitor slowly from a fixed voltage source via a large resistance, representing a constant current source, and can discharge the capacitor rapidly, the actions taking place in response to voltages of either polarily.